Cutting of glass is conventionally accomplished by using mechanical tools. Typically, the glass is first scored, such as by using a scoring tool (e.g. a sharp carbide wheel for example) that creates a score, or median crack, in the glass, and incidentally, substantial damage to the glass at the cut edge. However, alternative processes exist that use CO2 laser radiation at a wavelength of 10.6 μm to heat the glass and create tensile stress via a temperature gradient to produce a score, a less damaging process. During laser scoring, a small initial flaw is formed on a surface of the glass near one of its edges to generate a median crack (also known as a partial vent or, simply, a vent). The vent is then propagated by a laser light formed into a beam that traverses across the surface of the glass followed by a cooling area produced by a cooling nozzle. Heating of the glass with a laser beam and quenching it immediately thereafter with a coolant creates a thermal gradient and a corresponding stress field that is responsible for the propagation of the vent. When the score is completed, bending or shear stress is then applied to the glass that causes the vent to complete its propagation through the thickness of the sheet. In either case, however, separation of the glass sheet is a two step process consisting of creating a score, and then applying a stress to the glass (e.g. a bending stress) to propagate the vent and separate the sheet, sometimes referred to as “score and snap”. In some processes, a second laser beam may be used to apply thermal stress to the glass that completes the separation process.
A problem with conventional techniques for separating sheets of glass using a laser relates to the coefficient of thermal expansion (CTE) of the glass as it is initially scored. Conventional laser scoring techniques have used glasses with relatively high CTE'S, e.g., Corning Incorporated's Code 1737 LCD glass which has a CTE (0-300° C.) above 37×10−7/° C. More recent glasses, e.g., Corning's EAGLE2000® and EAGLE XG™ glasses, have lower CTE'S. Higher CTE'S, such as that of Code 1737 glass, translate into higher tensile stresses during heating which, all other things being equal, means that it is easier to laser score such glasses at higher speeds. The lower CTE's of the more modern glass substrates used by the LCD industry result in much lower scoring speeds when conventional laser scoring technology is used, further prolonging such two step methods. Finally, the two step process can be challenging to achieve good edge quality.
It would be beneficial to provide a single step process for separating glass sheets that can provide a clean edge (minimal damage to the glass) with minimal residual stress with a faster cycle time.